PALEOECOLOGY OF THE END-SKULLROCKIAN (TREMADOCIAN; EARLY ORDOVICIAN) TRILOBITE MASS EXTINCTION

Two major trilobite mass extinctions have been recognized in the Early Ordovician of Laurentia. The older event occurred at the transition between the Skullrockian and Stairsian regional ages and is characterized by the extinction of most trilobite genera and subfamilies. In the Ibex area, western Utah, the Skullrockian–Stairsian contact is well exposed in the uppermost part of the Red Canyon Member of the House Formation at sections in the southern House Range. Here, a moderately diverse pre-extinction fauna (typically 10–12 species) dominated by species of hintzecurines and Bellefontia-group asaphids, is replaced by a much lower diversity assemblage (5 species, two of which are exceedingly rare), dominated by three species currently assigned to Paraplethopeltis.

In the Red Canyon Member, the top of the Skullrockian comprises an interval of thin bedded calcisiltite that yields the youngest pre-extinction trilobites. Rhynchonelliform brachiopod shell beds that include rip-up clasts or a basal layer of intraclastic rudstone scour down into calciciltite succession. The brachiopod beds contain the first post-extinction trilobites, belonging to the basal Stairsian Paraplethopeltis genacurva Zone. In this interval species of Paraplethopeltis are extremely abundant, and a new species of dimeropygid and a species of the survivor genus Symphysurina are rare. The brachiopod beds can be identified at different localities across the Ibex area and are interpreted as recording the onset of a transgressive interval. Similar brachiopod blooms are associated with earlier stage-bounding extinctions in the Laurentian upper Cambrian.

The expression of the extinction is strikingly abrupt in the Ibex region. At Section B-TOP, diverse silicified trilobites typical of pre-extinction faunas (12 species) occur only 2 cm below the post-extinction brachiopod coquina. The causes of the extinction remain unclear. Previous geochemical work has suggested the upwelling of anoxic water into the carbonate platform as main cause of the extinction, though the Ibex sections contain no obvious sedimentologic or biotic evidence for anoxia.